With RC6 replication fails in IPv6-only environments. As a result a second DC does not obtain a RID pool after the join process completed. samba-tool drs showrepl shows replication errors in this case (WERR_BADFILE IIRC).
This is due to a trivial bug introduced during the major rework of dns_ex.c in commit c54fe86a63f73543eaf9b031e146d5f647c05830, which makes reply_to_addrs not count successful lookups of QTYPE_AAAA. The corresponding code is still present in 4.0.0.